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Stars form from cold molecular interstellar gas. Since this is relativelyrare in the local Universe, galaxies like the Milky Way form only a few newstars per year. Typical massive galaxies in the distant Universe formed starsan order of magnitude more rapidly. Unless star formation was significantlymore efficient, this difference suggests that young galaxies were much more gasrich. Molecular gas observations in the distant Universe have so far beenlargely restricted to very luminous, rare objects, including mergers andquasars. Here we report the results of a systematic survey of molecular gas insamples of typical massive star forming galaxies at <z>~1.2 and 2.3, when theUniverse was 40% and 24% of its current age. Our measurements provide empiricalevidence that distant star forming galaxies indeed were gas rich, and that thestar formation efficiency is not strongly dependent on cosmic epoch. Theaverage fraction of cold gas relative to total galaxy baryonic mass at z= 2.3and z=1.2 is ~44% and 34%, three to ten times higher than in today's massivespiral galaxies. The slow decrease between z~2 and 1 probably requires amechanism of semi-continuous replenishment of fresh gas to the young galaxies.

High molecular gas fractions in normal massive star forming galaxies in the young Universe